Three-dimensional measurement of radiant energy scattering characteristics with a turntable-type scanning interferometer

ABSTRACT

A method and apparatus for measuring radiant energy scattering characteristics of a test panel of dielectric material over a sphere are disclosed. The test panel is supported on a turntable at a position on the axis of the turntable. The turntable has a first antenna radially mounted thereon for rotation therewith and directed toward the position of the supported test panel. A second antenna is mounted for selective positioning along a circular arc within a plane that includes the axis of the turntable, which arc extends at least ninety degrees from said axis, wherein the second antenna is radially directed toward the position of a test panel that is axially supported on the turntable at all positions along said arc. The test panel support is rotated with respect to the turntable to selectively position the test panel at a predetermined incidence angle with respect to the first antenna. The turntable is rotated with respect to the second antenna to selectively position the test panel at an angle with respect to the axis of propagation of the second antenna that is equal to and opposite the predetermined incidence angle. Radiation transmitted between the first and second antenna is measured with the test panel in the supported position at each of a plurality of predetermined equal and opposite incidence angles with respect to the first and second antennas; and radiation transmitted between the first and second antennas is measured at each of the plurality of predetermined incidence angles when the test panel is not supported on the turntable. The second antenna is position at a plurality of positions along the circular arc at various ones of the predetermined incidence angles at which measurements are made as described above. Further measurements are made, both with the test panel in its supported position on the turntable and with test panel absent, of the radiation transmitted between the first and second antennas at the plurality of positions along the circular arc at the various ones of the predetermined incidence angles. The reflectance characteristics of the test panel are then determined by subtracting the measurements taken without the test panel in place from those taken with the test panel supported on the turntable.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation in part of application Ser. No. 970,527 for "Measurement of Radiant Energy Scattering Characteristics with a Turntable-Type Scanning Interferometer" filed Dec. 18, 1978.

BACKGROUND OF THE INVENTION

The present invention generally pertains to interferometer apparatus and methods, and is specifically directed to an improvement in interferometer methods and apparatus for measuring radiant energy scattering characteristics of a test panel. Radiant energy scattering characteristics include transmittance and reflectance characteristics.

In the prior art, an interferometer is used to measure transmittance and reflectance characteristics of a planar sheet of dielectric material at microwave frequencies. A test panel of the dielectric material is positioned in an anechoic chamber with two microwave antennas.

To measure the transmittance characteristics of the test panel, the two antennas are positioned so as to be directed toward each other along a common same axis of propagation, and the test panel is positioned between the two antennas along the common axis. The radiation between the antennas is measured with the test panel present to determine the percentage of incident radiation that is transmitted through the test panel.

The plane of the test panel is supported along a second axis that is normal to the common axis of propagation of the antennas and is selectively rotated about the second axis to enable the transmittance characteristics of the panel to be measured at a plurality of predetermined incidence angles.

To measure the reflectance characteristics of the panel both antennas are positioned on the same side of the test panel and are directed at the test panel with their respective axes of propagation at predetermined equal and opposite incidence angles. For each of a plurality of different predetermined incidence angles at which measurements are taken both antennas are repositioned and redirected. This method and apparatus for measuring the reflectance characteristics is obviously quite cumbersome.

SUMMARY OF THE INVENTION

The present invention provides an improved scanning interferometer apparatus and method for measuring radiant energy scattering characteristics of a test panel over a sphere.

The interferometer includes a turntable; a support on the turntable for supporting a test panel in a selected position along the axis of the turntable; a first antenna radially mounted to the turntable for rotation with the turntable and directed toward the selected position of the supported test panel; and a second antenna mounted for selective positioning along a circular arc within a plane that includes the axis of the turntable, which arc extends at least ninety degrees from such axis, wherein the second antenna is radially directed toward the selected position of a said supported test panel at all positions along the circular arc. Preferably, the support for the test panel is selectively rotatable with respect to the selected position on the turntable for enabling a supported test panel to be positioned at selected incidence angles with respect to the axis of propagation of the first antenna; and the test panel support is selectively translatable with respect to the turntable for enabling the plane of incidence of the supported test panel to be axially positioned on the turntable.

The method of the present invention for measuring radiant energy scattering characteristics of a test panel over a sphere includes the steps of (a) supporting the test panel on the turntable at a position on the axis of the turntable; (b) rotating the test panel with respect to the turntable to selectively position the test panel at a predetermined incidence angle with respect to the first antenna; (c) rotating the turntable with respect to the second antenna to selectively position the test panel at an angle with respect to the axis of propagation of the second antenna that is equal to and opposite the predetermined incidence angle; (d) measuring radiation transmitted between the first and second antennas with the test panel in the supported position at each of a plurality of predetermined equal and opposite incidence angles with respect to the first and second antennas; (e) measuring radiation transmitted between the first and second antennas at each of the plurality of predetermined incidence angles when the test panel is not supported on the turntable; (f) positioning the second antenna at a plurality of positions along the circular arc at various ones, but not necessarily all of the predetermined incidence angles at which measurements are made in accordance with steps (d) and (e); (g) measuring radiation transmitted between the first and second antennas with the test panel in its supported position with the second antenna at the plurality of positions along the circular arc at the various ones of the predetermined incidence angles; and (h) measuring radiation transmitted between the first and second antennas with the second antenna at the plurality of positions along the circular arc at the various ones of the predetermined incidence angles when the test panel is not supported on the turntable.

By scanning the measurements over the second circular arc for each predetermined angle of incidence of the test panel with respect to the axis of propagation of the first antenna, the transmitted and reflected energy distributions are accurately determined and any endfire radiation caused by guided waves is thereby located.

The measurements that are made with the test panel removed also are taken continuously as the turntable rotates continuously.

By also making the measurements with the second antenna at the plurality of positions along the first mentioned circular arc at the various ones of the predetermined incidence angles three dimensional measurements can be made of the radiant energy scattering characteristics of the test panel. These three-dimensional measurements enable determination of the forward and backward endfire radiation patterns and of the grating lobes caused by any periodic physical properties of the test panel, such as corrugations.

Other features of the present invention are discussed in the description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial perspective view and partial schematic block diagram showing the scanning interferometer of the present invention.

FIG. 2 is an enlarged view of a portion of the interferometer that is shown in perspective in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, a preferred embodiment of the scanning interferometer of the present invention includes a turntable 10, a support 11 for a test panel 12, a first antenna 14, a second antenna 15, a signal source 16, a receiver 17 and a data logger 18. The turntable 10, support 11, the first antenna 14 and the second antenna 15 are located within an anechoic chamber (not shown.) The turntable 10 and support 11 are covered by absorbing material 20 (as shown in FIG. 2 by broken lines.) The signal source 16, receiver 17 and the data logger 18 are located outside the anechoic chamber. The turntable 10 is supported on a rotatable shaft 22, which is journaled in a bearing (not shown) within a turntable base 23. A motor (not shown) is positioned in the base 23 and coupled to the shaft 22 for continuously rotating the turntable 10.

The scanning interferometer is adapted for measuring the reflectance and transmittance characteristics of a test panel of a dielectric sheet material at microwave frequencies.

The support 11 is supported on the turntable 10 by columns 25. A lathe cross feed 27 is centered on the support 11 for axially supporting a test panel 12 on the turntable 10. The lathe-cross feed has a channel for holding the test panel. The panel is translated by operation of manual control knobs 29 and 30, which control two micrometer drives and enable the test panel to be translated orthogonally in quarter-wavelength increments so as to enable the test panel 12 to be axially positioned on the turntable 10. The test panel 12 also may be rotated in the lathe cross feed to be disposed at a predetermined angle with respect to the axis of propagation of the first antenna 14.

The first antenna 14 is radially mounted on the turntable 10 by means of a section of waveguide pipe 32 for rotation with the turntable 10. The waveguide 32 is coupled to a waveguide 34, which extends from the signal source 16, by means of a waveguide rotating joint 35 which is positioned within the shaft 22 of the turntable 10. The waveguide rotating joint 35 enables the first antenna 14 to rotate with the turntable 10.

The first antenna 14 is directed so that is axis of propagation intersects the axis of the turntable 10.

The second antenna 15 is directed so that its axis of propagation also intersects the axis of the turntable 10. The second antenna 15 is coupled to the receiver 17 by a coaxial cable 37. The second antenna 15 is connected to the coaxial cable 37, by a section of waveguide pipe 39, which is secured to a carriage 40. The carriage 40 is mounted for selective positioning along a circular arc defined by a metal rail 41. The metal rail 41 is supported by a wooden frame 43. The frame 43 is disposed so that the circular arc defined by the rail 41 is within a plane that includes the axis 45 of the turntable 10. The circular arc extends at least ninety degrees from the axis 45. The second antenna 15 is radially directed toward the selected position of a supported test panel 12 at all positions along the circular arc 43. The metal rail 41 is graduated to indicate different angular positions.

The receiver 17 also is connected to the signal source 16 by a waveguide loop 38. Microwave signals provided by the signal source are provided to the first antenna 14 via waveguides 34 and 32 and are transmitted by the first antenna 14. Microwave signals received by the second antenna 15 are provided to the receiver 17 via the waveguide 37. The receiver also receives reference signals from the signal source 16 via the waveguide loop 38, and is adopted for processing the signals received on cable 37 and waveguides 38 for measuring both the intensity and phase of the signals received by the second antenna 15 with respect to the signals transmitted by the first antenna 14. The waveguide loop 38 equalizes the lengths of the transmitted signal and the reference signal paths. A data logger 18 is connected to the receiver 17 for recording the measured intensity and phase data.

To make measurements of the reflectance and transmittance characteristics, first the test panel 12 is rotated in the lathe cross feed 27 to a predetermined angle with respect to the axis of propagation of the first antenna 14. Then the turntable 10 is rotating continuously by the motor while microwave signals are transmitted by the first antenna 14 and received by the second antenna 15. Phase and intensity measurements are continuously recorded by the data logger 18 as the turntable 10 rotates. The procedure is repeated for each predetermined incidence angle which is changed by rotating the test panel 12 in the lathe cross feed 27 to a new selected position.

Transmittance characteristics measurements are recorded when the first and second antennas 14, 15 are on opposite sides to the test panel 12; and reflectance characteristics measurements are recorded when the first and second antennas 14, 15 are on the same side of the test panel 12.

Three-dimensional measurements are enabled by positioning the second antenna 15 at a plurality of positions along the circular arc defined by the rail 41 at various ones (but not necessarily all) of the predetermined incidence angles at which measurements are made as described above. Further measurements are then made of the radiation transmitted between the first antenna 14 and the second antenna 15 at the plurality of positions above the circular arc at the various ones of the predetermined incidence angles. Such further measurements are made both with rest panel 12 in its supported position on the turntable 10 and with the test panel absent.

Reflectance characteristics measurements also are taken and recorded with the test panel 12 covered by a metallic plate. The metallic plate data serves to normalize the phase and intensity based upon the assumption that the reflectance magnitude for the plate is "one." The lathe cross feed 27 is used to translate the metallic plate so that it is axially positioned on the turntable 10.

Data measurements are also recorded with the test panel absent as the turntable is rotated.

The transmittance and reflectance characteristics are determined from the measured data as follows:

For transmittance characteristics, let the measured radiation field with the panel present be E_(T). Although the phase of E_(T) is directly observable, the determination of |E_(T) | requires assuming that intensity is proportional to |E_(T) |². The data are normalized to the incident (no panel) value E_(I). Again intensity is assumed to be proportional to |E_(I) ². The transmittance (or complex transmission co-efficient) is

    T=E.sub.T /E.sub.I.

The power transmittance is |T|², and the insertion phase delay is the argument (or phase) of T.

Determination of the reflectance characteristics requires subtracting the direct field E_(D), i.e. the field measured with no panel present. Let E_(R) represent the field with panel present and E_(M) the field with the metal plate present. The field E_(D), E_(M) and E_(R) are complex-valued. The subtraction gives E_(R) ' the field reflected by the panel after subtracting the field coupled directly between the first and second antennas as

    E.sub.R '=E.sub.R -E.sub.D.

For the field with the metal plate, the subtraction of the directly coupled field gives

    E.sub.M '=E.sub.M -E.sub.D.

Reflectance (or complex reflection co-efficient) is

    R=E.sub.R '/E.sub.M '.

The power reflectance is |R|². The phase of the reflectance is the argument of R.

Approximate values of |R|² (but not the phase of R) also can be obtained without the metal plate data by normalizing the measured radiation field to the value measured when the axes of propagation of the first and second antennas are aligned. However, the method that uses the data with the metal plate gives more accurate results, and it is preferable because it gives the phase of R.

To determine the power absorptance |A|² assume that the quantities |R|² and |T|² are the only other wave mechanisms. Energy conservation provides that

|A|² =1-|R|² -|T|²

The above equation is approximate because |R|² and |T|² omit any energy radiated by the test panel as guided waves. 

We claim:
 1. A scanning interferometer, for enabling measurement of radiant energy scattering characteristics of a test panel, comprisinga turntable; support means on the turntable for supporting a test panel in a selected position along the axis of the turntable; a first antenna radially mounted to the turntable for rotation with the turntable and directed toward the selected position of a said supported test panel; and a second antenna mounted for selective positioning along a circular arc within a plane that includes the axis of the turntable, which arc extends at least ninety degrees from said axis, wherein the second antenna is radially directed toward the selected position of a said supported test panel at all positions along said arc.
 2. A scanning interferometer according to claim 1, wherein the support means are selectively rotatable with respect to the selected position on the turntable for enabling a said supported test panel to be positioned at selected incidence angles with respect to the axis of propagation of the first antenna.
 3. A scanning interferometer according to claim 2, wherein the support means are selectively translatable with respect to the turntable for enabling the plane of incidence of said supported test panel to be axially positioned on the turntable.
 4. A scanning interferometer according to claim 3, comprising a lathe cross feed for rotating and translating the support means to selectively position said supported test panel.
 5. A scanning interferometer according to claim 4, comprising a waveguide rotating joint for mounting the first antenna to the turntable for rotation with the turntable.
 6. A scanning interferometer according to claim 1, comprising a waveguide rotating joint for mounting the first antenna to the turntable for rotation with the turntable.
 7. A scanning interferometer according to claims 1 or 6, further comprising:a motor coupled to the turntable for continuously rotating the turntable.
 8. A method of measuring radiant energy scattering characteristics of a test panel, comprising the steps of(a) supporting the test panel on a turntable at a position on the axis of the turntable, wherein the turntable has a first antenna radially mounted thereof for rotation therewith and directed toward the position of the supported test panel; (b) rotating the test panel with respect to the turntable to selectively position the test panel at a predetermined incidence angle with respect to the axis of propagation of the first antenna; (c) rotating the turntable with respect to a second antenna which is mounted for selective positioning along a circular arc within a plane that includes the axis of the turntable, which arc extends at least ninety degrees from said axis, wherein the second antenna is radially directed toward the supported position of the test panel at all positions along said arc, to selectively position the test panel at an angle with respect to the axis of propagation of the second antenna that is equal to and opposite the predetermined incidence angle; (d) measuring radiation transmitted between the first and second antennas with the test panel in said supported position at each of a plurality of predetermined equal and opposite angles with respect to the first and second antennas; (e) measuring radiation transmitted between the first and second antennas at each of said plurality of predetermined incidence angles when the test panel is not supported on the turntable; (f) positioning the second antenna at a plurality of positions along said circular arc at various ones of the predetermined incidence angles at which measurements are made in accordance with steps (d) and (e); (g) measuring radiation transmitted between the first and second antenna with the test panel in said supported position with the second antenna at the plurality of positions along said circular arc at the said various ones of the predetermined incidence angles; and (h) measuring radiation transmitted between the first and second antennas with the second antenna at the plurality of positions along said circular arc for at the various ones of the predetermined incidence angles when the test panel is not supported on the turntable.
 9. A method according to claim 8, whereinstep (c) comprises continuously rotating the turntable so that the incidence angle of the test panel with respect to the axis of propagation of the second antenna continuously varies; and step (d) further comprises continuously measuring radiation transmitted between the first and second antennas as the turntable rotates to determine the angular distribution of energy reflected by and transmitted through the test panel.
 10. A method according to claim 9, whereinstep (e) comprises continuously rotating the turntable and continuously measuring radiation transmitted between the first and second antennas as the turntable rotates to determine the angular distribution of energy transmitted between the first and second antennas over a second circular arc within the test panel supported on the turntable.
 11. A method according to claim 8, whereinstep (e) comprises continuously rotating the turntable and continuously measuring radiation transmitted between the first and second antennas as the turntable rotates to determine the angular distribution of energy transmitted between the first and second antennas over a second circular arc without the test panel supported on the turntable.
 12. A method according to claims 8 or 9, further comprising the step of:(i) measuring radiation transmitted between the first and second antennas at each of said plurality of predetermined incidence angles when the test panel supported on the turntable has known reflectance characteristics.
 13. A method according to claim 12, wherein the test panel in step (i) is a metallic plate. 